1. Field of the Invention
The present disclosure generally relates to integrated circuits, and, more particularly, to integrated circuits including transistors having a ferroelectric dielectric in addition to other transistors, such as logic transistors and/or input/output transistors.
2. Description of the Related Art
Integrated circuits typically include a large number of circuit elements which include, in particular, field effect transistors. The circuit elements in an integrated circuit may be electrically connected by means of electrically conductive metal lines formed in an interlayer dielectric material. The electrically conductive metal lines may be provided in a plurality of interconnect layers that are stacked on top of each other above a substrate in and on which circuit elements, such as field effect transistors, and other circuit elements, such as capacitors, diodes and resistors, are formed. Metal lines in different interconnect layers may be electrically connected with each other by means of contact vias that are filled with metal.
Integrated circuits may include nonvolatile memory. In some types of nonvolatile memory, so-called ferroelectric transistors (FeFETs) may be employed. Ferroelectric transistors may include a gate electrode that is formed above a channel region provided between a source region and a drain region. A layer of a ferroelectric material is arranged between the gate electrode and the channel region. An electrical conductivity of the channel region of the ferroelectric transistor may be controlled by an electrical field that acts on the channel region. In addition to an electrical field that is created by applying a gate voltage to the gate electrode of the ferroelectric transistor, an electrical field caused by a ferroelectric polarization of the layer of ferroelectric material arranged between the gate electrode and the channel region may also act on the channel region of the ferroelectric transistor.
Depending on the direction of the ferroelectric polarization of the ferroelectric dielectric, the electrical field created by the ferroelectric polarization of the ferroelectric dielectric may have a substantially same direction as the electrical field created by the application of the gate voltage to the gate electrode, or the electrical field created by the ferroelectric polarization of the ferroelectric dielectric and the electrical field created by the application of the gate voltage to gate electrode may have substantially opposite directions.
If both electrical fields have substantially the same direction, a threshold voltage that needs to be applied to the gate electrode for switching the ferroelectric transistor into the electrically conductive state (for ferroelectric transistors being N-channel transistors, the transistor is switched into the electrically conductive state by applying the gate voltage) may be reduced, and the electrical conductivity of the channel region that is obtained when a particular gate voltage greater than the threshold voltage is applied may be increased. If both electrical fields have opposite directions, the threshold voltage of the ferroelectric transistor may be increased, and the electrical conductivity of the channel region that is obtained when a particular gate voltage greater than the threshold voltage is applied may be reduced.
The ferroelectric polarization of the ferroelectric dielectric may be influenced by applying a programming voltage between the gate electrode and the channel region. For example, the programming voltage may be applied to the gate electrode, and the source region, the drain region and, optionally, the body of the ferroelectric transistor may be maintained at mass potential. The programming voltage may be positive or negative, depending on the desired direction of the ferroelectric polarization of the ferroelectric dielectric. The ferroelectric polarization of the ferroelectric dielectric may be maintained even if the programming voltage is no longer applied. Thus, a bit of data may be stored in the ferroelectric transistor, wherein a first polarization direction of the ferroelectric dielectric may be identified with a logical 0, and a second polarization direction of the ferroelectric dielectric may be identified with a logical 1.
For reading the stored bit of data from the ferroelectric transistor, a gate voltage may be applied between the gate electrode and the source region of the ferroelectric transistor, wherein the gate voltage applied during the reading of the bit of data is typically lower than the programming voltage, so that the ferroelectric polarization of the ferroelectric dielectric is substantially not changed. Then, the electric current flowing through the ferroelectric transistor may be measured for determining the direction of the ferroelectric polarization of the ferroelectric dielectric.
For some applications, it may be desirable to form ferroelectric transistors and field effect transistors of other types on a same semiconductor substrate. For example, U.S. Patent Publication No. 2013/0270619 discloses a method wherein a high-k dielectric layer is formed above a first active region and a second active region so as to serve as a ferroelectric layer. The high-k dielectric layer is removed from above the first active region. The high-k dielectric layer is preserved above the second active region. A first electrode structure is formed above the first active region and a second electrode structure is formed above the second active region.
The present disclosure provides semiconductor structures and methods for the manufacturing thereof which provide an improved integration of ferroelectric transistors on a same semiconductor structure as other types of transistors.